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l'HOTON In,: 6878Sania tau bvd. Précision Beam Profiling BfiSÊSE www. p h o ion - i ne. co m Measuring Pulsed Beams with a Slit-based Profiler Puise Rates, Power, and Damage Considérations Although the NanoScan was designed originally to measure continuons wave (CW) laser beains, many lasers are operated in the pulsed mode. Measuring thèse pulsed beams has generally required the use of a CCD array profiler. This is a reasonable solution for low power lasers in the IJV and visible wavelength range, but thèse will require extenial atténuation. Once the lasers leave the UV-VTS range, array caméras become extremely expensive. Although low frequency pulsed lasers operating in the IHz to 1000Hz range have no real alternative to the array profiler, the NanoScan can measure kHz frequency lasers. The NanoScan profiler incorporâtes the "peak connect" algorithm and software-controlled variable scan speed on ail scanheads to enable the measurement of thèse pulsed lasers. The NanoScan is idéal for measuring Q-switchcd lasers and lasers operating with puise width modulation power (PWM) control. In the past few ycars, lasers with pico- and femtosecond puise durations have begim to be used in many applications. Although thèse lasers add some additional complication to the measurement techniques, the NanoScan is well suited to measure thein, too. We will discuss the measurement of ail thèse types of pulsed lasers below. PWM Lasers Many lasers, especially CO? lasers, use puise width modulation (PWM) to control the power level of the laser. This is not true, pulsed opération, but rather a réduction of the duty cycle to lower the average power, The beam opérâtes as if it were CW, and many operators do not even realize that the laser is pulsing. However, when attempting to measure a PWM laser with a scanning slit profiler, it must be treated as a pulsed laser source. To use the pulsed mode of the NanoScan the laser's puise frequency must be at least several kHz, and the combination of the frequency and beam size must provide a sufficient number of puises across the beam to generate a meaningful profile. Eight lo ten puises are a reasonable minimum PWM lasers usually operate around 10kHz. The relationship of the beam size and frequency is a fairly simple mathematical model The NanoScan drum speed is software controllcd from 1.25Hz to 20Hz. Thcrc aie two available drum sizes for the NanoScan; the standard head has a drum diameter of 42mm and the large aperture and high power heads use a larger drum with 84mm diameter. On the 42mmdrum at the 1.25Hz rotation rate the slits travel at around 116.6mm per second or 116.6fim per millisecond. At a 10kHz laser répétition rate, a 175pm beam would have 15 puises during the time that the slit was traversing it. This would provide enougli data to generate a meaningfiil profile. A smaller beam would require a faster puise rate, a Photon Inc. June 2007
